Cherry-pick #21775 to 7.x: Change cloud.provider from googlecloud to gcp

[kaiyan-sheng]

Cherry-pick of PR #21775 to 7.x branch. Original message:

What does this PR do?

This PR is to change cloud.provider value from googlecloud to gcp to match the value from add_cloud_metadata processor.

Also in ECS documentation, gcp is used as an example under cloud.provider field.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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Pinging @elastic/integrations-platforms (Team:Platforms)

[elasticmachine]

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Build stats

• Build Cause: [Pull request #21811 updated]

• Start Time: 2020-10-14T16:40:36.886+0000

• Duration: 67 min 42 sec

Test stats 🧪

Test	Results
Failed	0
Passed	602
Skipped	48
Total	650